SUBORDER D 



ARTIODACTYLA 



169 



two equally developed metacarpals (III. and IV.) always exceed the two 

 lateral ones (II. and V.) in length and strength, and this difference increases 

 as the extremities become fitted for swift running and leaping. In the 

 more specialised forms with four or two digits, sharp median ridges or 

 keels are developed on the distal articular surface of the anterior and 

 posterior metapodials. 



In the primitive artiodactyl fore-foot, the first metacarpal articulates 

 with the trapezium, the second with the trapezium, trapezoid and magnum, 

 the third with the magnum and unciform, and the fourth and fifth with 

 the unciform. This arrangement may be modified by the reduction of the 

 lateral digits (Fig. 226, A, B) ; in this kind of modification, which has been 



Fore-foot of tI, Oreodon; B, Xiphodon (with inadaptive reduction); C, Sus\ and D, Tragulus 

 (with adaptive reduction). 



defined by Kowalevsky as " inadaptive reduction," each metapodial inflexibly 

 maintains its place among the carpals pertaining to it ; hence the second 

 metacarpal articulates with the magnum, trapezoid and trapezium. In 

 "adaptive reduction" (Fig. 226, C, D) the two median metacarpals continue 

 subequal in height, their proximal articular surfaces broaden out, pusli the 

 lateral metapodials outward and backward, and thus extend their contact 

 with the carpus. In this arrangement, the second metacarpal is excluded 

 from the magnum, and articulates with but part of the trapezoid and with 

 the trapezium. According to Kowalevsky, this modification aflFords greater 

 strength to the foot, and qualifies its owner for a more successful struggle 

 for existence. In fact, all existing artiodactyls with reduced extremities 

 belong to the " adaptive " series. 



The pelvis is elongated and the ilium narrow. The femur lacks the third 

 trochanter. In more pidmitive forms the tibia and fibula are distinct and 

 well developed ; in more specialised types the fibula becomes reduced to a 

 splint-like proximal vestige, and a separate distal vestige which articulates 



